1. Field of the Invention
The invention relates to a method and device for substantially pinpointing the emission of a pulsed seismic source adapted to be lowered inside a well and to be coupled to the walls of the well by retractable anchorage means.
2. Description of the Prior Art
Many seismic prospection methods comprise the use of seismic sources lowered into wells or boreholes and activated successively at a plurality of positions at different depths. The waves emitted are received by receivers disposed in other wells or boreholes, which makes it possible to obtain a high power of resolution, or else disposed on the surface so as to restore oblique seismic profiles. The energy efficiency of seismic sources in wells is generally much better than that of sources operated on the surface, because they emit seismic waves under the weathered surface layer whose propagation characteristics are unfavorable, but it is largely conditioned by the means used for coupling them to the surrounding geological formations.
Percussion seismic sources generally comprise an elongated body suspended at the end of an electric suspension cable or a pipe string. The body is connected to coupling elements formed of arms, claws or mobile shoes which may be moved apart by actuating hydraulic cylinders and are applied to or driven into the walls of the well.
In other embodiments, the body of the seismic source is connected to an element of the packer type, well known by specialists, of the type having an expandable member formed of a central portion and a peripheral portion which can be expanded by rotating it with respect to the central portion. Packers may also be used comprising an enclosure defined by a deformable wall and which is expanded by injection of a pressurized liquid.
Different seismic sources associated with anchorage means are described in the French patent applications published under the numbers 2 597 214, 2 552 553, 2 558 601 respectively corresponding to U.S. Pat. Nos. 4,770,268, 4,773,501 and 4,648,478, and French patent application no. 2590994 corresponding to commonly-assigned copending U.S. application Ser. No. 936,618, filed Dec. 1, 1986.
Attempts have also been made to use in wells sources known in the field of sea or land seismic prospection such as implosion sources operating by fluid ejection or by sudden contraction of the volume of a closed enclosure, such as described in the French patent application published under the number 2 55 761 (corresponding to the U.S. Pat. No. 4,682,309) or else sparkers which generate pulses by the sudden discharge of an electric current between immersed electrodes or else explosion sources.
It has been discovered, when studying the behavior of such sources, that when a shock is produced at the time of triggering a source, a greater or lesser amount of energy, depending on the degree of coupling with the walls, is transmitted to the liquid column generally filling the well and in which it propagates.
The result is that the waves vibrating the liquid column in the well disturb the reception of the useful echos corresponding to signals transmitted directly to the walls by the source and complicate the processing of the data collected. Because of this loss of energy and its dispersion, the use of seismic well sources does not always give good results.